1. Field of the Invention
The present invention relates to apparatus and method for estimating a charge rate (abbreviated as SOC) of a secondary cell.
2. Description of the Related Art
Japanese Patent Application First Publications No. 2000-323183 published on Nov. 24, 2000 No. 2000-268886 published on Sep. 29, 2000, and a Japanese Paper titled “Estimation of Open Voltage and Residual Values for Pb Battery by Adaptive Digital Filter” announced by a Japanese Electrical Engineering Society (T.IEEE Japan), Volume 112-C, No. 4, published on 1992 exemplify previously proposed SOC estimating apparatus for the secondary cell. That is to say, since the charge rate (or called State Of Charge, i.e., SOC) of the secondary cell has a correlation to an open-circuit voltage V0 (cell terminal voltage when its power supply of the cell is turned off, also called electromotive force or open voltage), the charge rate can be estimated when open voltage V0 is obtained. However, a considerable time is needed until the terminal voltage is stabilized after the power supply is turned off (charge-and-discharge is ended). Hence, a predetermined time duration is needed from a time at which the charge-and-discharge is ended to determine an accurate open-circuit voltage V0. Therefore, since immediately after or during the charge/discharge time or charge-and-discharge, it is impossible to determine an accurate open-circuit voltage and the charge rate cannot be obtained using the above-described method. Nevertheless, to determine the open-circuit voltage V0, the open-circuit voltage V0 is estimated using a method disclosed in the above-described Japanese Patent Application First Publication No. 2000-323183.